Packaged microelectronic devices are used in cell phones, pagers, personal digital assistants, computers and many other products. A packaged microelectronic device typically includes a microelectronic die and an interposer substrate attached to the die. The die generally has an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The interposer substrate has a plurality of traces coupled to the bond pads on the die, numerous ball-pads that are electrically connected to the traces, and a solder-mask that covers the traces with openings over the ball-pads. The solder-mask is also referred to as the passivation or dielectric layer. An array of solder-balls is configured so that each solder-ball contacts a corresponding ball-pad to define a “ball-grid” array. Packaged microelectronic devices with ball-grid array connections generally have lower profiles and higher pin counts than conventional chip packages that use a lead frame.
Packaged microelectronic devices are typically mounted to circuit boards. When a ball-grid array is used to connect a packaged microelectronic device to a circuit board, the solder-balls are connected to contacts on the circuit board using surface mounting techniques. First, discrete volumes of a solder paste are deposited on the circuit board contacts. Next, the solder-balls are pressed into the solder paste on the contacts. As the packaged microelectronic device and the circuit board are pressed together, the solder-balls are surrounded by solder paste and touch, or are moved proximate to, the contacts on the circuit board. This assembly is then heated to reflow the solder so that the solder mechanically bonds and electrically connects the solder-balls to the contacts.
One concern about surface mounting ball-grid array packages to circuit boards is that electrical shorts can occur between traces on the interposer substrate and ball-pads and/or solder-balls that are adjacent to such traces. The shorting problem arises because the openings in the solder-mask layer may be too large. For example, an etching process is generally used to form the openings in the solder-mask, but the etching process may over etch the solder-mask and expose a portion of a trace adjacent to a ball-pad. When a portion of the adjacent trace is exposed, the conductive solder paste on the circuit board can create a short that renders the assembled microelectronic device and circuit board inoperable. The potential for shorts is especially problematic for high-density devices with high-density ball-grid arrays because the spacing between the traces and the ball-pads is very small.
It is very difficult for the manufacturer of the packaged microelectronic device to detect the problem of potential shorts. The difficulty arises because after etching and placement of the solder-balls on the ball-pads, the device will function properly in quality control testing even though an adjacent trace is exposed because quality control testing does not involve pressing the solder-ball onto a conductive paste. Thus, without a conductive paste to create a short, it is difficult for device manufacturers to detect faulty components that may have a short after being assembled to a circuit board.
The buyer of the packaged microelectronic device, however, will connect the device to a circuit board and discover the faulty components. If the device has an exposed trace in the opening with the solder-ball, solder paste can fill the gap between the exposed trace and the ball-pad and/or solder-ball when the solder-ball is placed in the solder paste. Alternatively, if the paste does not fill the gap initially, it might do so during the reflow process. The solder paste is conductive, and consequently, when the paste fills the gap between the exposed trace and the ball-pad and/or solder-ball, an electrical connection is created. In this case the electrical connection is a short. Accordingly, the packaged microelectronic device and the circuit board, which both worked properly before assembly, do not work properly after being assembled by the customer. After assembly it is more expensive to recover faulty parts than to replace them, and therefore, the faulty parts are generally discarded.
Because the shorting problem is most easily detected only after assembly, the problem often arises after the manufacturer has sold the packaged microelectronic device. Post-sale detection increases the magnitude of the problem for at least two reasons. First, the financial losses are greater because all of the manufacturing, inventory and marketing costs for the product have been incurred by the time the problem is detected. Yet, almost none of these costs can be recouped because the faulty parts are often discarded and replaced. Second, post-sale detection can damage customer relationships because defective products disrupt the customers' business and damage their reputation. Even if customers are compensated for their costs, the process is an inconvenience at the very least.
Manufacturers of packaged microelectronic devices want to produce products with high yields. Therefore, it is desirable to prevent shorting, or at the very least, detect the possibility of shorts early in the manufacturing process. One approach manufacturers use to prevent shorting is to improve the accuracy of the etching process. This approach, however, increases the cost of the etching process and is becoming more difficult as the density of microelectronic devices increases. Moreover, even if the accuracy of the etching process is improved, some traces may still be exposed and susceptible to shorting at assembly.